Injection molding is a common manufacturing practice. Injection molding is a cyclic process in which melted (plasticized) plastic is injected (forced) into a mold cavity or cavities, where it is held under pressure until it is removed in a solid state, basically duplicating the cavity of the mold. Various articles of commercial value, such as plastic bottles, toothbrushes, automotive parts, medial device, children's toys, etc., are made using well-known injection molding techniques.
One type of injection molding process, referred to as hot runner injection molding, generally involves melting a material, which is often a polymeric material or resin, then forcing the melt stream at high temperatures and pressures to flow through an injection manifold that communicates with one or more hot runner nozzles. The heated nozzles further guide the melt flow through a mold gate into one or more mold cavities. The melt cools in the shape of the mold cavity, which is opened to eject the finished part.
One method to control the flow of the melt stream into a mold cavity is by using a valve-gated injection molding apparatus. Usually a valve pin has a cylindrical or tapered front end and reciprocates between a retracted open position and an extended closed position, in which the front end is seated in a gate. In some applications, the valve pin functions in the reverse direction and closes in the retracted position.
Valve-gated mechanisms are, however, typically designed to open and close the gates in a binary fashion, i.e., the gate is either opened or it is closed without allowing for a partially-opened configuration in which the melt flow rate or amount is controlled through the gate.
In some manufacturing processes, the ability to control the melt stream (i.e., temperature and pressure of the melt) during the shot is highly desirable. Another instance in which control over the melt stream flow is desirable is when a number of parts are simultaneously molded in separate cavities. Typically, in this case feed systems are used to deliver the molten plastic from the injection unit to the separate cavities. In many or most injection molds, multiple branches and outlets are used in the feed system to deliver the melt to the separate cavities, so as to simultaneously form multiple articles or complex articles that require multiple flows of melt. Each mold cavity is fed from a common melt stream through one or more gates. In such a system, the mold cavities are not necessarily all the same size, such as when components of an interlocking assembly, such as cellular telephone housings, are simultaneously molded. Thus, the optimum pressure to fill and pack each cavity is different. This is because when the cavities are of different sizes and shapes and/or the melt travel time to each cavity is different, properties of the melt, e.g., viscosity, is affected. Thus, it is typically not possible to optimize pressure for each cavity by controlling an overall pressure of the injection molding machine.
In a multi-gated system in which a single mold cavity is fed melt through multiple hot runner nozzles and mold gates, a common manifold can serve all of the gates. In such a system a “knit line” or “weld line” may be formed at the interface where melt flowing from one gate meets melt flowing from another gate. Even though all of the gates are commonly fed, the ability to individually control the flow rate through each gate allows a designer to control the location of the knit line for structural or aesthetic purposes. Also, part quality attributes are significantly affected by the flow-rates and pressures in the injection stage of the molding cycle. Typical part defects such as hesitation, jetting, unbalanced filling, knit-lines, orientation, and race-tracking are influenced by flow rates in the filling stage. Typical part defects related to the packing stage dynamics are sink, flash, shrinkage, warpage, and residual stress. Any changes made to the velocity/pressure profile simultaneously affect the flow-rates and pressures in all the cavities in the mold.
Various methods exist or have been proposed in the art to provide flow control over the melt stream. One way to provide flow control is to individually re-tool the runner for every new product, but this is expensive and time-consuming. Alternatively, dynamic systems incorporating sensors and closed-loop control with multiple adjustable valve pins, or flow control rods, have been proposed. As a further alternative, passive systems have been utilized.
In one dynamic feed system utilizing a closed-loop system with pressure sensors, each valve pin extends through a nozzle body and is aligned with its own respective gate, and the gates are each fluidly connected to a common mold cavity or respective cavities. The valve pins are exposed to the high pressure melt through the length of the nozzle body. Pressure data measured by the pressure sensors is read at or near the injection point into the mold cavity. Each valve pin of the system is then dynamically adjusted by a computer according to the pressure data measured by the pressure sensors for the corresponding injection point.
Other dynamic feed systems have been proposed that utilize melt flow control rods positioned upstream of the gate that interact with a narrowed portion of the melt channel to control melt flow. Such dynamic feed systems impose additional costs in equipment, i.e., require a controller, pressure transducers/sensors, etc. in order to create the required closed-loop feedback system, as well as in operating costs due to the complexity of the systems. In addition, oftentimes a large portion of the pin, or control rod, is unsupported during operation and subject to deflection that may result in misalignment and reduced service life.
In another dynamic system, the manifold includes a “shooting pot.” A portion of the melt stream is diverted from the manifold melt channel into a separate compartment or “well.” An actuated ram is disposed within the well and can be positioned to seal the opening of the well. A valve-gated nozzle is located downstream of the well such that a flow of melt through a mold gate orifice is controlled by an actuated valve pin. When the melt stream is introduced into the manifold melt channel, the valve pin is seated within the mold gate orifice to prevent flow into a mold cavity. The well ram is located in a retracted position so that a volume of melt from the manifold may be diverted into the well and contained therein. To start the shot, a gating mechanism located upstream from the well closes the manifold melt channel, thereby preventing the introduction of new melt into the well. The valve pin of the nozzle is unseated from the mold gate orifice, and the ram is moved forward at a first velocity to force melt from the well into the nozzle melt channel and subsequently into the mold cavity. A system of pressure sensors measures the pressure in the system and compares that pressure reading to a target pressure profile. If greater pressure is required, the ram velocity is increased. Alternatively, if lesser pressure is required, the ram velocity is slowed. When the ram reaches its lowermost position, the mold cavity is full, and the valve pin is moved forward to close the mold gate. Through this manipulation of the ram velocity, the flow rate of the melt stream can be controlled. This control over the melt stream requires completely closing off one portion of the manifold melt channel in order to manipulate the melt stream in another portion thereof. In addition to imposing additional costs associated with the control of the rams and valve pins, the system also creates a secondary interruption in the melt flow. Such secondary interruptions may create additional imperfections in the molded parts.
In another system, a sliding spool valve includes at least one recess used to control the volumetric flow of melt and a separate gate closure portion. The valve is configured so that the melt flowing through the valve creates minimal axial force upon the spool thereby minimizing the force necessary to actuate the spool. The recess is configured primarily to control the volumetric flow of the melt so the size of the gate closure portion can be reduced. Both the upstream and downstream ends of the spool, except for the small gate closure portion, are exposed to atmospheric pressure. The valve is operated in either a preset profile or a dynamic fashion. For example, the preset profile mode can mean that an open loop or no loop system is used that has no feedback control and that uses a preset operating profile for controlling pressure. In contrast, the dynamic operation mode can mean that a closed loop system is used that has feedback control and that uses a continuously changing operating profile for controlling pressure, which is based on a real-time sensed condition. In the preset profile mode, the position of the spool valve is profiled in the filling stage of the molding process to provide a varying volumetric flow rate. Alternatively, in the dynamic mode, the location of the spool valve is controlled or the location is adjusted based on measurements of the melt pressure or load on the spool valve. Regardless of the mode of operation, this system requires that the spool valve and housing both have complicated geometries. In addition, means for venting a portion of the spool valve, which may be positioned deep within a mold block, must be provided. Furthermore, if the system is operated in the dynamic mode, it provides the additional shortcomings of the dynamic systems described above.
Various passive systems have been developed. In general, the passive systems commonly utilize a spring-loaded plunger that closes the gate of a nozzle when the plunger is in a retracted position and only operate in an open or closed position. The upstream end of the plunger has a projected surface area exposed to the melt that differs from a projected surface area on the downstream end of the plunger. At the beginning of an injection cycle, the pressure upstream of the plunger is generally higher than the pressure of the melt downstream, which causes the plunger to extend, thereby opening the gate. As the pressure downstream of the plunger in the cavity increases, the differing surface areas, in combination with the spring force, creates a net force differential on the plunger causing it to retract into its closed position. In such systems, the plunger is generally unsupported along its length and the plunger and/or spring are exposed to the melt. As a result, these passive systems may also be prone to reduced service lives.